1. Field of the Invention
This invention relates to electromechanical actuators, and particularly to actuators utilizing piezoelectric elements to rotate positioning shafts associated with optical elements.
2. Description of the Prior Art
The bench setups used in experimental optics frequently incorporate individual optical elements, such as lenses, filters, mirrors, radiation sources and radiation detectors, which are mounted on adjustable supports. Such supports are capable of both lateral and vertical adjustment to properly position the element within the optical system. Additionally, some form of tilt (axial orientation) adjustment is provided for properly orienting the element with respect to the optical axis of the system. While the particular form of the tilt adjustment varies according to the nature of the optical element and the precision required, a common form of adjustable mount is a three screw device, such as the Model 9809 sold by New Focus, Inc. of Mountain View, Calif., and described in U.S. Pat. No. 5,140,470, entitled "Optical Mounting Apparatus" issued to Francis S. Luecke and assigned to New Focus, Inc., the assignee of this invention. This mount, and other similar mounts sold by New Focus, Inc., has three 1/4-80 adjustment screws arranged in a triangular configuration for tilting the optical element held by the mount. The adjustment screws in this device are provided with knurled knobs for manual adjustment.
These mounts have been highly successful and are widely used in experimental optical setups. They provide excellent stability and accommodate ease of adjustment. Nevertheless, it would be desirable to avoid even the slight deflection of the optical element which results from manual adjustment of the mount. Further, certain experimental optical setups may occupy a large space, and make manual adjustment of the mounts located in the interior portions of the optical bench more awkward than desirable. In addition, it is usually desirable to make dynamic adjustments on the system; that is, while the system is in actual operation. This introduces an element of danger with systems incorporating lasers, since inadvertent exposure to laser radiation may permanently damage eyesight or other parts of the human body.
Despite the disadvantages inherent in the use of manual adjustment screws, the use of electrically driven actuators has been limited by cost, size and stability considerations. These limitations have limited the use of remotely controlled actuators to particular optical elements, wherein the optical element and the actuator are interrelated and the actuator is totally dedicated to the single element. While this approach is frequently satisfactory in a finished optical system, it is unduly cumbersome and expensive in experimental setups.
It has been recognized that piezoelectric systems are well suited for mechanically driving the positioning shafts in an optical system. For example, U.S. Pat. No. 4,622,483 to Staufenberg and Hubbell is descriptive of a system which utilizes piezoelectric elements to alternatively clamp and drive a mechanical element such as shaft.
U.S. Pat. No. 4,727,278 to Staufenberg describes a piezoelectric multi-axis positioner for rotation of a sphere which supports an optical element. The tilt systems described therein are limited to spherical mounts, and do not lend themselves to use with universal mounts such as the New Focus Model 9809.
Still another piezoelectric driven system is shown in U.S. Pat. No. 4,831,306 to Staufenberg and Hubbell. In this system, a piezoelectric element mounted within an annular housing is energized in a fashion, whereby an engaging member is driven with a first polarity signal to cause engagement with an output shaft, and then driven with a second polarity signal to withdraw the member from engagement with the output shaft.
Piezoelectric devices have been used for other purposes such as that described by L. Howald, H. Rudin and H.-J. Guntherodt in "Piezoelectric inertial stepping motor with spherical rotor", Review of Scientific Instruments 63 (8) August 1992 p. 3909-3912. The publication describes a system in which a plurality of inertial piezoelectric actuators are used to position a polished steel sphere which may support an optical element. In the system described by the publication, the actuators are driven slowly in one direction and abruptly in the opposite direction. Friction causes the polished ball to follow the slow actuator movement but inertia prevents the ball from following the abrupt actuator movement.
A publication by John D. Skipper, "Piezoelectric Traction Motor Delivers High Torque, High Power at Low Speed" in PCIM, June 1992, p. 36-39, describes a piezoelectric motor having a rotary output shaft. This publication describes the difficulty of coupling piezoelectric devices to a rotary output shaft due to the very small mechanical movement of the piezoelectric device.